How to make an object only collides with the terrain?
It's simple: use collision masks. Download the example file here.

The concept is pretty easy:




Collision Group are the collision 'layers' that the object belongs to. Multiple can be selected.
Collision Mask are the collision 'layers' that the object can collide with. Multiple can be selected.


For example, if Ball2 belongs to Collision Group 2, but can only collide with objects on Collision Group 1 (through the use of Collision Mask), it will not collide with itself.